The present invention relates to an intensity controlling circuit device for an LED-array head used for printers or facsimile machines.
A light emitting diode (LED) is used for projecting a light beam so as to expose a photosensitive material of a printer drum provided in a printer. Intensity of the LED may be controlled using an intensity controlling circuit device 6 shown in FIG. 1. In the device 6 shown in FIG. 1, a few thousands to a few ten thousands of LEDs 1-1 to 1-n may be used. P-channel type MOS (PMOS) transistors 2-1 to 2-n, which are slave transistors, are connected to anodes of the respective LEDs 1-1 to 1-n. A gate of each transistor 2-1 to 2-n is connected to a power source 8 via respective PMOS transistors 7-1 to 7-n. The gate of each transistor 2-1 to 2-n is also connected to an output of the intensity controlling circuit device 6 via respective transmission gates 5-1 to 5-n. Outputs of AND circuits 9-1 to 9-n are connected to gates of the PMOS transistors 7-1 to 7-n and control terminals of the transmission gates 5-1 to 5-n. The AND circuits 9-1 to 9-n are connected to outputs of latch 4. The latch 4 latches binary image data, which is stored in a shift register 3, in synchronization with a shift clock signal supplied to the shift register 3. Each of the AND circuits 9-1 to 9-n supplies the binary image data to the respective LEDs 1-1 to 1-n according to an enable signal supplied to an input thereof.
In the above-mentioned circuit, data according to the binary image data latched by the latch 4 is output from the AND circuits 9-1 to 9-n while the enable signal supplied to the AND circuits 9-1 to 9-n is in a high level state. The slave transistors 2-1 to 2-n are selectively operated to supply an electric current I2 to the respective LEDs 1-1 to 1-n so that the selected LEDs 1-1 to 1-n emit light beams.
A construction of the intensity controlling circuit 6 is also shown in FIG. 1. The intensity controlling circuit device comprises a PMOS transistor 11, an N-channel type MOS (NMOS) transistor 12, a resistor 13 and an operational amplifier 14. A source of the PMOS transistor 11 is connected to the power source 8. A gate of the PMOS transistor 11 is connected to the PMOS transistors 2-1 to 2-n. A gate of the NMOS transistor 12 is connected to an output of the operational amplifier 14. A reference voltage (Vref) is applied at a positive terminal of the operational amplifier 14. A voltage applied to a point A in the figure is supplied to a negative terminal of the operational amplifier 14. A connecting point, indicated by B in the figure, between the PMOS transistor 11 and the NMOS transistor 12 is connected to the inputs of the transmission gates 5-1 to 5-n. The resistor 13 is connected between the NMOS transistor 12 and the ground. The NMOS transistor 12, the resistor 13 and the operational amplifier 14 together comprise a constant current generating means.
The operational amplifier 14 controls a voltage applied to the gate of the NMOS transistor 12 so that the voltage at the point A is equal to the reference voltage Vref. Since the point A is grounded via the resistor 13, a current I1 (=Vref.div.resistance of the resistor 13) flows to the resistor 13. The current I1 flows into a drain of the NMOS transistor 12 via a drain of the PMOS transistor 11.
Since the PMOS transistor 11 and each of the PMOS transistors 2-1 to 2-n comprises a current mirror circuit, a gate voltage of the PMOS transistor 11 is applied to a gate of each of the PMOS transistors 2-1 to 2-n via the respective transmission gates 5-1 to 5-n. Accordingly, if each of the PMOS transistors 2-1 to 2-n has the same transistor size with the PMOS transistor 11, a current equal to the current I1 flowing in the PMOS transistor 11 flows to the PMOS transistors 2-1 to 2-n. Accordingly, a current flowing to each of the LEDs 1-1 to 1-n can be controlled by the current flowing in the PMOS transistor 11. It should be noted that the PMOS transistor 11 is referred to as a master transistor.
Additionally, Japanese Laid-Open Patent Application No. 63-240168 discloses, with reference to FIG. 3, a circuit for adjusting an intensity of a light beam emitted by each LED provided in an LED-array. In the LED-array, a plurality of LEDs are arranged along a line. This circuit is provided for adjusting the intensity of light beam emitted by each of the LEDs so as to correct variation in intensity of the light beam due to tolerance of each LED.
In the circuit shown in FIG. 3 of Japanese Laid-Open Patent Application No. 63-240168, an LED driving transistor and an AND circuit are needed for each of the LEDs. This construction causes an increase in size of the LED-array chip. Additionally, reliability of the LED-array chip is reduced due to an increase in the number of gates provided in the LED-array chip.
For example, a conventional LED-array head has 64 LEDs arranged in a 5 mm square chip (LED-array chip). In an LED-array head used for A1 size printer, approximately 300 LED-array chips are arranged along a line. The above-mentioned intensity controlling circuit 6 formed in the LED driving circuit chip is also provided to each of the LED-array chips. In the above-mentioned circuit construction of the LED-array head, all the transistors and AND circuits comprising together the LED driving circuit must be operated in a normal condition. Accordingly, an inspection time for the LED-array head may be increased, and possibility of defect of the transistors and AND circuits may be increased, and thus reliability of the LED-array chip is decreased.
However, since LEDs and LED driving circuits have recently been manufactured using a wafer manufacturing process, the tolerance in the LEDs in the LED-array and the LED driving circuits can be almost zero. Therefore, recently, there has been no need to adjust the intensity of the light beam emitted by each of the LEDs in the LED-array.
On the other hand, while the variation in the light beam emitted by the LEDs in a single LED-array is reduced, variation of the intensity of the light beams due to tolerance in the LED-array chips and the LED driving circuit chips still exists in the order of a few percent to a few tens percent. This variation in intensity of the light beams causes a deterioration in printing quality.